Meta-stable austenitic stainless steels of improved hot workability

ABSTRACT

CHROMIUM-MANGANESE-NICKEL STAINLESS STEELS OF LOW, CONTROLLED NITROGEN CONTENT HAVING A PREDOMINANTLY AUSTENITIC STRUCTURE AS HEAT TREATED, AND PARTIALLY TRANSFORMABLE TO MARTENSITE ON COLD WORKING, AND POSSESSING IMPROVED HOT WORKABILITY TOGETHER WITH GOOD CORROSION RESISTANCE.

April 6, 1971 Filed July 19, 1968 K. G. BRICKNER ET AL 3,573,900 META-STABLE AUSTENITIC STAINLESS STEELS OF IMPROVED HOT WORKABILITY 6 Sheets-Sheet 1 ./4 -./5% a. Steels .//-./2 96 6. Steels ./l 6. (T-ZO/l I 1 l l l l l l .02 .04 .06 .08 .IO -./2 .I4 .16

Nitrogen Content, Weight Percent INVEN TORS.

KENNETH G. BRIG/(IVER 8 DAV/D 6. LUDW/GSON A t torney Number of Twists to Failure at 2300" 1.

April 6, 1971 K. s. BRICKNER ETAL 3,573,900

META-STABLE AUSTENITIC STAINLESS STEELS OF IMPROVED HOT WORKABILITY Filed July 19, 1968 e Sheets-Sheet z 80 .aas/v.

pet-.040 N.

.ora/v. A

A 07-09% .oso/v.

' .//0/v. I50 N. (7-20!) m t/501v. 20.. .//-./5%N.

t I L l l l l l l .02 .04 .06 .08 ./o .12 .14 ./6 ./8

Carbon Content, Weight Percent INVENTORS.

KENNETH a. BRICK/V67? a DAV/D c. LUDW/GSON r Attorney April 6, 1971 G -BR|CKNER ET AL 3,573,900

META-STABLE AUSTENITIC STAINLESS STEELS 0F IMPROVED HOT WORKABILITY 6 Sheets-Sheet 5 Filed July 19, 1968 cmukm 9235 N E co umegm 0 0 0 0 v a a 7 6 5 m 3 v 3 h I g H e f s v e I 0 s n w. 0 n T n e 0 r w n r a M. s n N m .m U E Y 0 0 0 0 0 0 0 0 5 4 2 0 8 6 4 2 l I l I G uss 5 5 2. m 8 3 Carbon Content, Weight Percent Attorney April 6, 1971 K. aamcmza ETAL 3,573,900

META-STABLE AUSTENITIC STAINLESS STEELS OF 6 Sheets-Sheet 4 IMPROVED HOT WORKABILITY Filed July 19. 1968 Chromuim Canreni, Weigh! Percenl INVENTOR KENNETH 6. BRICK/V67? 8 DA V/D C LUDW/GSOIV 51 A,

Attorney April 6, 1971 BRlCKNER ETAL 3,573,900

META-STABLE AUSTENITIC STAINLESS STEELS OF IMPROVED HOT WORKABILITY Filed July 19. 1968 6 Sheets-Sheet i 5 Ultimate Tensile Strength 0 =0.02-0 .03 96 N Steels A =0.0.9 76 N Steels a, /20 S Q Q i E Iongatian Q -i Q Q! i E m e a w g i 5 3 w E 5 60 5 E.

Q Q & Yield 4o Strength Chroma/m Content, Weight Percent INVENTOR KfN/VETH 6. BRIG/(NEH 8 DAVID C LUDW/GSO/V Attorney United States Patent 01 lice 3,573,900 Patented Apr. 6, 1971 US. Cl. 75-128 3 Claims ABSTRACT OF THE DISCLOSURE Chromium-manganese-nickel stainless steels of low, controlled nitrogen content having a predominantly austenitic structure as heat treated, and partially transformable to martensite on cold working, and possessing improved hot workabiilty together with good corrosion resistance.

RELATED APPLICATIONS This application is a continuation-in-part of co-pending application Ser. No. 378,932, filed June 29, 1964, now abandoned, in the names of Kenneth G. Brickner and David C. Ludwigson.

BACKGROUND OF THE INVENTION It is well known in the production of stainless steels that certain elements, such as nickel, manganese and nitrogen, act as austenite promoters, serving to preserve the high temperature austenitic structure when the metal is cooled to room temperature.

Consequently, alloying elements, as nickel and manganese are frequently added in substantial amounts in the production of austenitic stainless steels. Nitrogen may also be intentionally added for such purpose, or the nitrogen inadvertently included in the steel melt in the course of production, as in air melting, may be utilized to advantage in this regard. US. Pat. No. 3,149,965, directed to the provision of stably austenitic stainless steels for automotive valve fabrication, is illustrative of such use of these austenite stabilizing elements.

However, in absence of careful control of the melting process, the amount of nitrogen dissolved in the steel as a result of melting varies widely from residual amounts on the order of 0.06% (as noted in the above-mentioned patent) or less, to the much larger amounts possible, e.g. to 0.2% or more for 14-15% Cr steel melts at 2700 F. (see, for example, H. A. Wriedt et al., Trans, AIME, 1961, vol. 221, pp. 532-535; reproduced in part in The Making, Shaping and Treating of Steel, 8th edition, 1964, page 293. United States Steel Corporation) depending on the particular steel-making practice used and the nature and amount of other alloy elements, such as nickel and, especially, manganese, which are also present.

It is, of course, now well known steel-making practice to utilize nitrogen, in relatively large amounts, for example, 025 Weight percent maximum, in chromium-nickelmanganese steels, to produce austenitic steels, e.g. AISI 'Iypes 201 and 202. By such Austenitic steels, common usage denotes. and we, in this connection intend to denote, stainless steels which are not heat hardenable and which are predominantly austenitic as commercially heat treated. Such steels do, however, undergo a partial transformation to martensite upon cold working.

Other stainless steels are known to the prior art which are completely or substantially completely stably austenitic, not only on heat treatment, but also during cold working. Such stably austenitic stainless steels find application, for example, in end uses requiring a fully nonmagnetic (austenitic) structure even after fabrication by cold working, as, for example, in electrical generator retaining rings. Exemplary of these latter steels are those described in German Pat. No. 652,472, or, more recently, in US. Pat. Nos. 2,876,096 and 2,909,425. In such steels, the amounts and proportions of the ferrite promoters, as chromium, and the austenite promoters, as carbon, nickel, manganese and nitrogen, are such as to effectively preelude any significant austenite transformation On heating or cold working.

In certain metal fabricating industries, such as the automotive and domestic appliance industries, there is need for a low cost austenitic stainless steel having good formability (low yield strength and high elongation), high ultimate tensile strength, and good corrosion resistance. Chromium-manganese-nickel steels, having sufficient nitrogen to insure heat treated austenitic structure, have heretofore been proposed as lower cost substitutes for the higher cost chromium, nickel austenitic steels. However the poor hot workability of such substitute steels otfsets the lower alloy cost by increasing production costs.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide low cost austenitic chromium-manganese-nickel stainless steels of controlled, low nitrogen content, and which steels not only provide good formability and tensile strength, together with good corrosion resistance, but which also possess enhanced hot workability and hence entail less diflicult and less costly production.

Such object, together with further advantages as hereinafter described, are provided by steels having the following broad, intermediate and preferred compositions:

Composition, weight percent Interme- Element Broad diate Preferred Carbon 0.l1-0.20 0.12-0.l7 0 14-017 Nitrogen 0.0150.08 0.0150.06 0 015-0. 04 Chromium 14-19 14-17 14-15. 25 Manganese 5 5. 5. 5-6. 5 Nickel 4. 5-5. 25 Phosphorous, maximum .0 0.03 Sulfur, maximum 0.04 0.04 0.03 Silicon, maximum 1.0 1.0 .35-0. Molybdenum Up to 0.50- Up to 0.50 0.25-0.50

DESCRIPTION OF THE DRAWINGS The foregoing, and other objects and advantages of the invention will be more fully understood by reference to the following description and drawings, wherein:

FIGS. l-6 are graphs showing the desired relation between carbon and nitrogen contents and certain physical properties of several embodiments of the invention.

DESCRIPTION OF THE PREFERRED 'EMBODIMENT TABLE I Mn P S S1 Ni Cr Mo +=Residual.

Steels of the foregoing compositions were processed to bar and sheet product by hot working at 2200 to 2250 F. and, in some cases, followed by cold rolling.

Hot twist tests were conducted on certain of the steels with the results given in the following Table II. Hot twist tests determine the hot-working characteristics of metals. The apparatus consists essentially of a furnace for heating and maintaining a specimen (%-inch diameter by about 12-inch length) at the desired temperature, a motor for rotating the specimen, chucks to grip the specimen and a counter to record the number of twists. In performing the test, one end of the specimen is fastened to the driving end of the motor while the opposite end is secured against rotation. A series of different test temperatures are used and the test is continued until the specimens break. The number of twists required to break or fracture the specimen is a reliable indication of the hot workability of the steel at the particular temperature.

The results of hot twist tests on certain of the steels of Table I are given in the following Table H.

TABLE II Number of twists to failure at the indicated temperature 2,000 F. 2,100 F. 2,200 F. 2,300 F.

* Two tests.

TABLE III.-LONGITUDINAL ROOM-TEMPERATURE TENSILE PROPERTIES Yield strength Tensile Elongation in 0.2% ofl'set, strength, 2 inches p.s.i. p.s.l. percent 4 It will be noted that the steels of this invention have elongations in 2 inches of 50% or better. This is markedly better than the 20 to 25% elongation of AISI Type 430 steel frequently used for domestic appliance and automotive trim applications.

Table IV gives some comparative data on Steel I of this invention and certain AISI Type 430, 304 and 316 steels of the following compositions:

AISI types 0 Mn 1? S Si Cu N1 Cr Mo N TABLE IV.-CORROSION RESISTANCE Corrodent: 10 percent nitric acid, F., 24 hours:

Wt. loss, mg./sq. decimeter-day Steel I 1.8 AISI Type 430(1) 3.7

Corrodent: Mufiier condensate, F., 1000 hours:

Max. pit depth, in.

Steel I 0.007 AISI Type 430(2) 0.017 AISI Type 304 0.016 AISI Type 316 0.004

From the foregoing, it is seen that Steel I of this invention has corrosion resistance better than AISI 430 in 10% nitric acid. It also has better resistance than AISI Types 304 and430 in muffier concentrate and closely approaches that of the much more expensive AISI Type 316 in its resistance to the latter corrosive media.

Referring now in detail to the drawings:

FIG. 1 is a graph illustrating the effect of nitrogen content, at various carbon contents, on the hot workability of chromium-manganese-nickel-nitrogen steels. FIG. 1 shows the drastic and desirable effect in increasing hot workability by holding the nitrogen content to low levels, i.e. under 0.08 and preferably under 0.06 or 0.04%, especially in the higher carbon content steels, i.e. those having .14 to .17% carbon.

FIG. 2 is a graph illustrating the effect of carbon content, at various nitrogen contents, on the hot workability of chrominum-manganese-nickel-nitrogen steels. FIG. 2, as does FIG. 1, illustrates the interrelationship of carbon and nitrogen and the effect of those elements on hot workability of chromium-manganese-nickel-nitrogen steels. This figure vividly illustrates the desirability of maintaining the nitrogen content under about .08%, preferably under .04%, and the necessity of maintaining relatively high carbon content, i.e. over .11% and preferably over .14%. The combination of high carbon in the range .11 to .20, preferably .14 to .17, together with low nitrogen, i.e. .015 to .08, preferably .015 to .04, is readily seen from FIG. 2.

FIG. 3 is a graph showing the eifect of carbon content upon yield and tensile strength and percent elongation in low, i.e. .03 to .04%, 17% chromium steels containing chromium-manganese-nickel-nitrogen. As shown in FIG. 3, carbon has relatively little effect upon yield or tensile strength in the contemplated chromium-manganesenickel-nitrogen steels but at least about .11 and preferably at least .12 to .14% carbon is needed in order to obtain highest elongation.

FIG. 4 is a graph illustrating the effect of chromium upon hot workability of chromium-manganese-nickelnitrogen steels. As shown in FIG. 4, chromium, in range from about 14 to about 17%, has essentially no effect upon hot workability of the contemplated chromiummanganese-nickel-nitrogen steels.

FIG. is a graph showing the effect of chromium content and nitrogen content upon the yield and tensile strength and the percent elongation of chromium-manganese-nickel-nitrogen steels. FIG. 5 shows that yield strength of the contemplated steels is not substantially affected by increasing chromium content in the range from 14 to 19% but that ultimate tensile strength decreases with increasing chromium content within this range. FIG. 5 also shows that elongation increases with increasing chromium content when the nitrogen content is held in the desirable low range, e.g. about 0.02 to 0.03%.

FIG. 6 is a graph showing the effect of hot Working temperature upon the hot workability of chromium-manganese-nickel-nitrogen steels. As shown in FIG. 6, the steels of the invention having relatively high carbon contents, together with relatively low nitrogen contents, for example steel 9 containing 17% carbon and .035 nitrogen, and steel 28 containing .11% carbon and .018% nitrogen, exhibit excellent and superior hot workability throughout the hot working temperature range and particularly at the high end thereof.

While We have shown and described several specific embodiments of our invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of our invention, as defined in the appended claims.

We claim:

1. Meta-stable austenitic stainless steel consisting essentially of by weight percent:

Carbon 0.11 to 0.20 Nitrogen 0.015 to 0.08 Chromium 14 to 19 Manganese 5.5 to 8 Nickel 4.5 to 6 Phosphorous (max.) 0.04 Sulfur (max.) 0.04 Silicon (max.) 1.0 Molybdenum Up to 0.50

with the remainder iron, and incidental impurities, wherein said C, N and Cr are balanced; with N being present on the low side of its range, when C is under about 0.14%, and wherein N and C are on the high side of their respective ranges when Cr is over about 17%, thereby providing a steel with superior hot workability, and a superior ductility as evidenced by an elongation in 2 inches in excess of 50%.

2. Meta-stable austenitic stainless steel consisting essentially of by weight percent:

with the remainder iron, and incidental impurities, wherein said N is limited to a maximum of about 0.04% when C is under about 0.14%, thereby providing a steel with superior hot workability, and a superior ductility as evidenced by an elongation in 2 inches in excess of 3. Meta-stable austenitic stainless steel consisting essentially of by weight percent:

Carbon 0.14 to 0.17 Nitrogen 0.015 to 0.04 Chromium 14 to 15.25 Manganese 5.5 to 6.5 Nickel 4.5 to 5.25 Phosphorous (max.) 0.03 Sulfur (max.) 0.03 Silicon 0.35 to 0.60 Molybdenum 0.25 to 0.50

with the remainder iron, and incidental impurities, said steel being characterized by superior hot workability, and ductility as evidenced by an elongation in '2 inches in excess of 50%.

References Cited UNITED STATES PATENTS 2,289,081 7/1942 Shortell 123X 3,149,965 9/1964 Jennings 75l28 FOREIGN PATENTS 652,472 10/ 1937 Germany.

OTHER REFERENCES Enduro 201 Steel, Alloy Digest, Stainless Steel, File Code: 58-58, Engineering Alloys Digest, Inc., Upper Montclair, NJ. 2 pages, received July 8, 1957.

L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner US. Cl. X.R.

75128P, 128N, 128W UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pat t N 3 573 900 Dated April 6 1971 Inventor) Kenneth G Brlckner et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4 line 2 under heading "Si", "38" should read .38 line 4 under heading "Cr", "78 .1" should read 18 .1 line 50 "chrominum" should read chromium Signed and sealed this 7th day of December 1971 (SEAL) Attest:

EDWARD M. FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Pater 

